In a mobile communication system (such as Personal Handyphone System: abbreviated as PHS hereinafter) rapidly developed in recent years, a base station is practically used which enables transmission and reception with directivity by performing so-called adaptive array processing using an array antenna formed with a plurality of antennas to attain a good communication quality.
With this adaptive array processing by the array antenna, a communication system called PDMA (Path Division Multiple Access) system or SDMA (Space Division Multiple Access) system also becomes feasible wherein a spatial multiple access of wireless terminals (terminals or personal stations) of a plurality of users to a radio base station (base station or cellular station) is possible by spatially dividing a time slot of the same frequency to enhance utilization efficiency of radio wave frequency.
According to such adaptive array technology, an up-link signal from an antenna on each user terminal is received by an array antenna on a base station, and is separated and extracted with reception directivity by the adaptive array processing. On the other hand, a down-link signal from the base station to the terminal is transmitted from the array antenna with transmission directivity to the antenna on the terminal.
Since such adaptive array processing is a well-known technology and is described in detail in, for example, reference 1: “Chapter Three: MMSE Adaptive Array” (pp. 35–49) in “ARRAY ANTENNA NIYORU TEKIOUSHINGOUSYORI (Adaptive Signal Processing by Array Antenna)” by Nobuyoshi Kikuma (Sci Tech Press), the operating principles thereof will not be described herein.
In the following description, the base station performing down-link transmission directivity control for the terminal using such adaptive array processing is referred to as “an adaptive array base station”.
As the communication system of the above-described PHS, TDMA system is adopted, wherein a frame (5 ms) consisting of respective four slots (one slot: 625 μs) for transmission and reception is used as a base unit. SDMA system uses the same frame structure. This communication system of PHS is standardized, for example, as “a second generation cordless telephone system”.
FIG. 13 is a schematic diagram showing a structure of a signal transmitted and received between a terminal and a PHS base station.
A signal of one frame is divided into eight slots and, for example, the former four slots are used for reception while the latter four slots are used for transmission. Each slot includes 120 symbols.
In the PHS system, a prescribed number of frames such as 20 frames, each having such a structure, form one cycle. That is, in the PHS system, one base station communicates with a terminal within its service area using a control channel once in every prescribed number of frames such as once in every 20 frames, and the terminal obtains information such as whether the terminal can make a call at the present position.
In the frame structure shown in FIG. 13, a set of one slot for reception and one slot for transmission is allocated to the control channel while the remaining three sets of slots are respectively allocated to traffic channels for three users, for example, in the first frame of the 20 frames cycle.
In each of the other 19 frames, the set of slots corresponding to the control channel is kept empty, and the remaining three sets are successively allocated to the traffic channels for three users.
During a control procedure of synchronization establishment in the PHS system, a link channel is first established by the control channel, followed by interference wave (U wave: Undesired wave) measurement processing and setting of a traffic condition with an allocated channel, and then a speech starts. Such a procedure is disclosed in detail in a standard of PHS, that is, a second generation cordless telephone system standard RCR STD-28 (issued by Association of Radio Industries and Businesses).
FIG. 14 shows such a traffic sequence flow of the PHS system. Brief description thereof will be given referring to FIG. 14.
First, a link channel establishment request signal (LCH establishment request signal) is transmitted from a PHS terminal to a base station using a C channel (control channel: CCH). The PHS base station detects (carrier sense) an empty channel (empty traffic channel: empty T channel) and, using the C channel, transmits to the PHS terminal a link channel allocation signal (LCH allocation signal) specifying the empty T channel.
At the PHS terminal, a measurement (U wave measurement) is performed to determine if the specified T channel receives an interference wave signal having power equal to or higher than a certain value, based on the link channel information received from the PHS base station. When the interference wave signal having power equal to or higher than the certain value is not detected, that is, when the specified T channel is not used by another PHS base station, a synchronous burst signal is transmitted to the base station using the specified T channel, and then the base station transmits the synchronous burst signal back to the terminal to complete the establishment of synchronization.
On the other hand, when the interference wave signal having power equal to or higher than the certain value is detected, that is, when the specified T channel is used by another PHS base station, the PHS terminal repeats the control procedure by again transmitting the link channel establishment request signal.
In the PHS system, a connection of a communication channel between a terminal and a base station is performed as such using a channel with a small interference wave which can obtain a good communication characteristic.
When the control channel CCH is used for communication only in one frame of 20 frames cycle, for example, and is kept empty in the remaining 19 frames as described above, however, utilization efficiency of radio wave decreases. The efficiency is especially low in a condition of heavy traffic.
Though it is possible to use slots for the control channel in the above-mentioned remaining 19 frames for speech communication, as the normal communication using the control channel is performed once in every 20 frames, communication data of the traffic channel allocated to the slot is interrupted once in every 20 frames (once in every 100 ms) in this situation. This causes noise, and thus a speech quality is degraded.
Therefore, an object of the present invention is to provide an apparatus for a radio base station, and a method and a program of controlling communication channel allocation which can enhance the utilization efficiency of radio wave in a mobile communication system wherein a plurality of terminals are connected to a base station.